1. Field of the Invention
The present invention relates to a voltage measurement device for detecting a very low voltage applied to a local portion of a device to be measured.
2. Related Background Art
As conventional methods for measuring a very low voltage signal applied to a local portion of a device to be measured, the first method for measuring a voltage signal by contacting a probe of an oscilloscope to an electrical circuit to be measured, the second method for optically measuring a voltage signal using an E-O probe consisting of an electro-optic crystal, and the like are known. As an example of the second measurement method, a method disclosed in Japanese Patent Laid-Open No. 156379/1991 is known. This reference discloses a technique for detecting an electrical signal applied to an electrical circuit to be measured without bringing the E-O probe into contact with a device to be measured.
However, in the first measurement method, since a measurement is performed by bringing the probe into direct contact with an electrical circuit to be measured, the constant of the electrical circuit undesirably changes. Furthermore, a signal waveform upon transmission is distorted due to the distributed constant of a transmission line for connecting the probe and a main body, or external noise is superposed on an electrical signal on the transmission line, thus deteriorating the S/N ratio. Owing to these problems, it is difficult to measure a true electrical signal by the measurement method using the oscilloscope.
It is difficult to measure a plurality of points at the same time by the second measurement method. Furthermore, due to a very small electro-optic crystal, and a large number of optical elements, it is difficult to align the optical elements.